Method and Device for Processing Crankshaft and Burnishing Roller for Crankshaft

ABSTRACT

A pair of burnishing rollers formed in a roughly disk shape are linearly disposed oppositely to each other on both sides of the shaft part of a crankshaft. The crankshaft is rotated about its axis by the rotatingly driving action of a rotating driving source (M). The pair of burnishing rollers are displaced in a direction apart from each other while the shaft part of the crankshaft is held by the outer peripheral surfaces of the pair of burnishing rollers to apply a rolling to the crankshaft.

TECHNICAL FIELD

The present invention relates to a method of and an apparatus for machining (processing) a crankshaft to burnish a shank of the crankshaft for vehicle engines or the like, and a crankshaft burnishing roller for burnishing a shank of a crankshaft by rollingly pressing an outer surface of the shank of the crankshaft.

BACKGROUND ART

Heretofore, it is known to burnish a shank of a crankshaft for engines for increased productivity and improved surface properties of the shank.

For example, Patent Document 1 discloses a technical idea for making the machining surface pressure of the burnishing roller applied from a burnishing roller to a shank lower at both ends of the burnishing roller than at the center thereof in order to prevent both ends of the shank of the crankshaft from being rounded.

Patent Document 1 discloses a roller burnishing apparatus for burnishing a crankshaft with a burnishing roller, the apparatus including a pair of support rollers by which a shank of the crankshaft is rotatably supported, a burnishing roller having a dimension substantially identical to the width of the shank, and a backup roller for engaging the burnishing roller to press the burnishing roller in rolling contact with the shank.

Patent Document 2 discloses a roller burnishing method and apparatus for burnishing a workpiece having a centrally concave shape that can easily be burnished with a roller. Convex regions of the workpiece are burnished to desired surface roughness by an ordinary burnishing roller, whereas the central concave region is used as an oil reservoir to allow itself to be supplied appropriately with lubricating oil for increased durability and operability.

The burnishing apparatus disclosed in Patent Document 2 comprises a pair of support rollers by which a pin of a crankshaft is rotatably supported, a support member supporting the support rollers, a burnishing roller for pressing the pin of the crankshaft remotely from the support rollers, and a bracket by which the burnishing roller is rotatably supported.

Patent Document 3 reveals a roller burnishing tool including a tool shank base of circular cross section which has a plurality of ridges disposed on an outer surface thereof. At least one of the ridges has side edges extending across a generatrix along the circumferential direction of the tool base. Each of the ridges is shaped such that the sum of the widths of the ridges as measured in the axial direction of the tool base remains essentially the same at any circumferential positions on the tool base.

According to the technical ideas disclosed in Patent Documents 1, 2, fillets for clearing the tool are formed at the axial ends of the shank of the crankshaft. The ends near the fillets are compressed when they are pressed by the burnishing roller to prevent round corners from being formed. The technical ideas disclosed in Patent Documents 1, 2 are based on the premise of a crankshaft with fillets.

If a shank of a crankshaft free of fillets is burnished by a burnishing roller as is the case with the present invention, then since it is not necessary to take into account the formation of round corners, it is requested that the conventional machining method be simplified to improve surface properties of the shank.

The burnishing apparatus disclosed in Patent Documents 1, 2 are complex in structure and highly costly to manufacture because they need a plurality of components including the pair of support rollers (support members), the backup roller, etc.

The roller burnishing tool disclosed in Patent Document 3 is advantageous in that if a shank (a surface be burnished) of a crankshaft has an oil hole defined therein, for example, then when the shank is burnished, the ridges of the roller burnishing tool tend to fall into the oil hole, damaging the surrounding edge of the opening of the oil hole.

Patent Document 1: Japanese Laid-Open Patent Publication No. 06-190718

Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-276121

Patent Document 3: Japanese Laid-Open Patent Publication No. 04-115866

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide an apparatus for machining a crankshaft to improve surface properties of a shank of the crankshaft which is free of fillets.

A major object of the present invention is to provide a method of machining a crankshaft to improve surface properties of a shank of the crankshaft which is free of fillets.

Another object of the present invention is to provide an apparatus for machining a crankshaft, the apparatus being simplified in structure and less costly to manufacture.

Still another object of the present invention is to provide a crankshaft burnishing roller for improving surface properties of a shank of a crankshaft which is free of fillets and for appropriately burnishing the shank even if an oil hole is defined therein.

According to the present invention, a pair of burnishing rollers, each substantially disk-shaped, is placed in opposite relation to each other across a shank of a crankshaft therebetween, and when the crankshaft is rotated about its own axis, the shank of the crankshaft is gripped and rollingly pressed by outer circumferential surfaces of the burnishing rollers.

According to the present invention, therefore, the shank has surface properties improved by a machining method which is simpler than heretofore. As a result, the surface properties of the shank of the crankshaft which is free of fillets are improved, and a crankshaft machining apparatus is simplified in structure and less costly to manufacture.

According to the present invention, furthermore, the burnishing rollers for rollingly pressing the shank of the crankshaft have outer circumferential surfaces having a plurality of ridges having respective crests axially spaced at equal intervals, and the crests of adjacent two of the ridges are spaced from each other by a distance which is greater than the inside diameter of an oil hole. Therefore, even if the oil hole is present in a region to be burnished, the ridges do not fall into the oil hole, and the surrounding edge of the oil hole is not damaged.

According to the present invention, therefore, the surface properties of the shank of the crankshaft which is free of fillets are improved, and even if the oil hole is present in the region to be burnished, the shank can appropriately be burnished.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a general structure of a crankshaft machining apparatus according to an embodiment of the present invention;

FIG. 2 is a partial enlarged plan view of the crankshaft machining apparatus shown in FIG. 1;

FIG. 3 is a partial enlarged plan view of a crankshaft machining apparatus having a burnishing roller supported on its both sides;

FIG. 4 is an enlarged plan view, partly omitted from illustration, showing the manner in which a pair of burnishing rollers is displaced while gripping and rollingly pressing a shank of a crankshaft;

FIG. 5 is an enlarged plan view, partly omitted from illustration, showing the manner in which a pair of offset burnishing rollers is displaced;

FIG. 6 is an enlarged plan view, partly omitted from illustration, showing an outer circumferential surface of the burnishing roller which serves as a roll-pressing surface;

FIG. 7 is an enlarged plan view, partly omitted from illustration, showing the manner in which the burnishing roller is pressed into contact with the shank in a direction perpendicular to the axis of the shank;

FIG. 8 is an enlarged plan view, partly omitted from illustration, showing the manner in which the burnishing roller is pressed into contact with the shank obliquely at a predetermined angle from the direction perpendicular to the axis of the shank;

FIG. 9 is a plan view showing a general structure of another crankshaft machining apparatus incorporating burnishing rollers according to an embodiment of the present invention;

FIG. 10 is a partial enlarged plan view of the crankshaft machining apparatus shown in FIG. 9;

FIG. 11 is a partial enlarged plan view of a crankshaft machining apparatus having the burnishing roller shown in FIG. 9 which is supported on its both ends;

FIG. 12 is an enlarged plan view, partly omitted from illustration, showing a plurality of ridges on an outer circumferential surface of the burnishing roller shown in FIG. 9;

FIG. 13 is an enlarged plan view, partly omitted from illustration, showing the manner in which a pair of burnishing rollers is displaced while gripping and rollingly pressing a shank of a crankshaft;

FIG. 14 is an enlarged plan view, partly omitted from illustration, showing the manner in which a pair of offset burnishing rollers is displaced;

FIG. 15 is an enlarged plan view, partly omitted from illustration, showing the relationship between the ridges on the outer circumferential surface of the burnishing roller shown in FIG. 9 and an oil hole defined in the shank;

FIG. 16 is an enlarged plan view, partly omitted from illustration, showing the manner in which the burnishing roller shown in FIG. 9 is pressed into contact with the shank in a direction perpendicular to the axis of the shank; and

FIG. 17 is an enlarged plan view, partly omitted from illustration, showing the manner in which the burnishing roller shown in FIG. 9 is pressed into contact with the shank obliquely at a predetermined angle from the direction perpendicular to the axis of the shank.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1, the reference character 10 represents a crankshaft machining apparatus according to an embodiment of the present invention.

The crankshaft machining apparatus 10 is mounted on the turret (or rotary tool base) of a turret lathe (or a machining center) which is controlled by a numerical control system, not shown. The crankshaft machining apparatus 10 comprises first and second adapters 12 a, 12 b displaceable in the directions of three axes X, Y, Z which are perpendicular to each other, a pair of burnishing rollers 16 a, 16 b rotatably supported respectively on rotational shafts 14 of the first and second adapters 12 a, 12 b, and first and second holding mechanisms 22 a, 22 b for engaging respective both axial ends of a crankshaft 18 (hereinafter referred to as workpiece 18) and holding the workpiece 18 such that the workpiece 18 can be rotated about its own axis by a rotary drive source M.

The burnishing rollers 16 a, 16 b are disposed in mutually opposite relation to each other linearly perpendicular to the axis of the workpiece 18 across a shank 24, which is a region to be burnished, of the workpiece 18 (see FIG. 4), or are disposed in opposite relation to each other at respective positions that are offset in opposite directions by equal distances parallel to the axis of the workpiece 18 from a linear position perpendicular to the axis of the workpiece 18 (see FIG. 5).

Each burnishing roller 16 a (16 b) is of a substantially disk-like identical shape and has an outer circumferential surface 20 shaped like a single flat web. As shown in FIG. 6, the outer circumferential surface (roll-pressing surface) 20 of each burnishing roller 16 a (16 b) has a round corner 26 having an arcuate cross-sectional shape on one side surface, and a single slanted surface extending continuously from the round corner 26 to a corner 28 on the other side surface obliquely at an angle θ1 with respect to a line L parallel to the axis of the shank 24. The angle θ1 should preferably be set to about 1 degree.

The round corner 26 of each burnishing roller 16 a (16 b) may have a radius r2 of curvature smaller than a radius r1 of curvature of round corners 48 (to be described later) on the both axial ends of the shank 24 as the region to be burnished (r1>r2).

The first and second adapters 12 a, 12 b comprise respective slide members 32 displaceable in the direction indicated by the arrow A in unison with the burnishing rollers 16 a, 16 b against the spring forces of pressure control springs (to be described below), not shown, a pair of respective guide rods 34 a, 34 b for guiding the slide members 32, pressure control springs, not shown, for biasing the slide members 32 toward the workpiece 18, and respective knobs 36 for adjusting the spring forces of the pressure control springs (see FIG. 2).

When the operator grips the knobs 36 and turn the knobs 36 in certain directions, the pressure control springs are set to an initial set load (a preloading pressure, to be described later).

As shown in FIG. 2, a plurality of needle bearings 38 for absorbing radial forces on the burnishing rollers 16 a, 16 b are interposed between central inner portions of the burnishing rollers 16 a, 16 b and the rotational shafts 14, and a plurality of ball bearings 40 for absorbing thrust forces on the burnishing rollers 16 a, 16 b are interposed between the burnishing rollers 16 a, 16 b and bearing plates 39 coupled to the slide members 32. The burnishing rollers 16 a, 16 b are prevented from being dislodged from the rotational shafts 14 by nuts 42 fitted over the rotational shafts 14.

The adapters 12 a, 12 b may be of either the type in which the burnishing roller 16 a (16 b) is cantilevered on one side (see FIG. 2) or the type in which the burnishing roller 16 a (16 b) is supported on its both sides by arms 44 a, 44 b which face each other (see FIG. 3).

The crankshaft machining apparatus 10 according to the present embodiment is basically constructed as described above. Operation and advantages of the crankshaft machining apparatus 10 will be described below.

First, a process of manufacturing the crankshaft 18 will briefly be described below.

After a forged blank is machined on a lathe, oil holes are bored in the blank, for example, and then the shank 24 is burnished (plastically machined) by the burnishing rollers 16 a, 16 b of the crankshaft machining apparatus 10 which grip the shank 24 and are pressed in rolling contact with the shank 24. Then, the workpiece is heat-treated by gas nitriding (GSN) or high-frequency heating (HFQ) or the like, after which the workpiece is finished and finally inspected into a product.

As described above, the burnishing process based on the roll-pressing operation of the crankshaft machining apparatus 10 is performed as a process preceding the heat-treatment (GSN or HFQ) on the crankshaft.

Operation of the crankshaft machining apparatus 10 will be described below.

After the workpiece 18 is set and rotatably supported on the first and second holding mechanisms 22 a, 22 b, the knobs 36 are turned to adjust the spring forces of the non-illustrated pressure control springs to set the preloading pressure to a prescribed value. The preloading pressure refers to a pressing force applied to the shank 24 when the shank 24 as the region to be burnished is gripped and pressed by the burnishing rollers 16 a, 16 b that are disposed in mutually opposite relation to each other. Stated otherwise, the preloading pressure refers to a machining surface pressure which the shank 24 receives from the burnishing rollers 16 a, 16 b. The preloading pressure may be set to a level depending on the surface roughness which is required of the crankshaft 18.

It is assumed that when the burnishing rollers 16 a, 16 b are in their initial positions, they are spaced from each other by a predetermined distance radially from the shank 24 of the workpiece 18 and are waiting in opposite positions which are spaced a predetermined distance linearly perpendicular to the axis of the workpiece 18.

After the above preparatory stage, the rotary drive source M is energized to rotate the workpiece 18 about its own axis at a predetermined angular velocity. Then, based on a control signal from the non-illustrated numerical control system, the first and second adapters 12 a, 12 b are displaced toward the workpiece 18 in respective directions toward each other, and an axial central area of the shank 24 is gripped by the burnishing rollers 16 a, 16 b.

Since the workpiece 18 is rotating about its own axis, the burnishing rollers 16 a, 16 b are caused to rotate in respective opposite directions by the rotating workpiece 18. The shank 24 is burnished by the rotating burnishing rollers 16 a, 16 a as the outer circumferential surfaces 20 thereof rollingly press the surface to be burnished of the shank 24. According to the burnishing process, small surface irregularities, not shown, on the outer surface of the shank 24 are flattened to provide good surface roughness.

When the burnishing rollers 16 a, 16 b contact the shank 24, they should be brought into oblique contact with the surface to be burnished of the shank 24 (see FIG. 8), rather than being pressed against the shank 24 in a direction perpendicular to the axis of the shank 24 (see FIG. 7). The angle (θ2) at which the burnishing rollers 16 a, 16 b brought into oblique contact with the shank 24 may range from about 20 degrees to 45 degrees to avoid a dent 46 (see FIG. 7) which would otherwise be formed in a plastic surface if the burnishing rollers 16 a, 16 b are pressed perpendicularly against the shank 24.

After the burnishing rollers 16 a, 16 b are linearly positioned across the shank 24 therebetween and start rollingly pressing the shank 24, one of the burnishing rollers 16 a (16 b) and the other burnishing roller 16 b (16 a) are caused to slide in respective opposite directions (directions away from each other parallel to the axis of the crankshaft 18) to rollingly press the region to be burnished of the shank 24.

As described above, after the central area of the region to be burnished is gripped by the burnishing rollers 16 a, 16 b, the burnishing rollers 16 a, 16 b are displaced in respective directions away from each other to burnish the region to be burnished. Consequently, the burnishing time is shortened for higher efficiency.

Since the burnishing rollers 16 a, 16 b are caused to slide in respective opposite directions, the round corners 48 on the both axial ends of the shank 24 as the region to be burnished can also appropriately be rollingly pressed by the round corners 26 of the burnishing rollers 16 a, 16 b (see FIGS. 4 and 5). As the radius r2 of curvature of the round corner 26 of each burnishing roller 16 a (16 b) is smaller than the radius r1 of curvature of the round corners 48 on the both axial ends of the shank 24, the round corners 48 can appropriately be rollingly pressed.

The shank as the region to be burnished comprises pins and journals of the crankshaft 18.

As shown in FIG. 5, the burnishing rollers 16 a, 16 b may not be disposed in mutually opposite relation to each other linearly perpendicular to the axis of the shank 24, but may be displaced toward the shank 24 of the crankshaft 18 from respective positions that are slightly offset horizontally to grip the shank 24. Thereafter, the burnishing rollers 16 a, 16 b may be caused to slide in respective directions away from each other to burnish the region to be burnished.

According to the present embodiment, the burnishing rollers 16 a, 16 b are disposed in mutually opposite relation to each other across the shank 24 of the crankshaft 18, and while the crankshaft 18 is rotating, the shank 24 of the crankshaft 18 is burnished by being gripped and rollingly pressed by the burnishing rollers 16 a, 16 b. Consequently, the surface properties of the shank 24 of the crankshaft 18 which is free of fillets are improved, and the crankshaft machining apparatus is simplified in structure and less costly to manufacture.

FIG. 9 shows another crankshaft machining apparatus 110 incorporating the burnishing rollers 116 a, 116 b according to an embodiment of the present invention. Those parts of the crankshaft machining apparatus 110 which are identical to those of the crankshaft machining apparatus 10 are denoted by the identical reference characters, and will not be described in detail below.

The crankshaft machining apparatus 110 includes a pair of burnishing rollers 116 a, 116 b rotatably supported on the respective rotational shafts 14 of the first and second adapters 12 a, 12 b which are displaceable in the directions of three axes X, Y, Z which are perpendicular to each other.

The burnishing rollers 116 a, 116 b are disposed in mutually opposite relation to each other linearly perpendicular to the axis of the workpiece 18 across the shank 24, which is a region to be burnished, of the workpiece 18 (see FIG. 13), or are disposed in opposite relation to each other at respective positions that are offset in opposite directions by equal distances parallel to the axis of the workpiece 18 from a linear position perpendicular to the axis of the workpiece 18 (see FIG. 14).

Each burnishing roller 116 a (116 b) is of a substantially disk-like identical shape and has an outer circumferential surface for rollingly pressing the shank 24. The outer circumferential surface of each burnishing roller 116 a (116 b) has first through fourth ridges 120 a through 120 d having respective crests spaced at equal intervals along the axis of the burnishing roller (see FIG. 12). The crests of adjacent two of the first through fourth ridges 120 a through 120 d are spaced from each other by a distance S which is greater than the inside diameter T of an oil hole 126 defined in the shank 24 (S>T) (see FIG. 15).

The number of the ridges is not limited to four. Rather, the outer circumferential surface of each burnishing roller 116 a (116 b) may have at least three ridges. The outer surface of each of the first through fourth ridges 120 a through 120 d for contacting the shank 24 of the crankshaft 18 has a round corner 28 having an arcuate cross-sectional shape. The round corner 28 has a radius r2 of curvature which is smaller than the radius r1 of curvature of the round corners 48 on the both axial ends of the shank 24 (see FIG. 15).

The adapters may be of either the type in which the burnishing roller 116 a (116 b) is cantilevered on one side (see FIG. 10) or the type in which the burnishing roller 116 a (116 b) is supported on its both sides by arms 44 a, 44 b which face each other (see FIG. 11).

The crankshaft machining apparatus 110 incorporating the burnishing rollers 116 a, 116 b according to the present embodiment is basically constructed as described above. Operation and advantages of the crankshaft machining apparatus 110 will be described below.

After a predetermined preparatory process, the rotary drive source M is energized to rotate the workpiece 18 about its own axis at a predetermined angular velocity. Then, based on a control signal from the non-illustrated numerical control system, the first and second adapters 12 a, 12 b are displaced toward the workpiece 18 in respective directions toward each other, and an axial central area of the shank 24 is gripped by the burnishing rollers 116 a, 116 b.

Since the workpiece 18 is rotating about its own axis, the burnishing rollers 116 a, 116 b are caused to rotate in respective opposite directions by the rotating workpiece 18. The shank 24 is burnished by the rotating burnishing rollers 116 a, 116 a as the first through fourth ridges 120 a through 120 d on the outer circumferential surfaces 20 thereof rollingly press the surface to be burnished of the shank 24. According to the burnishing process, small surface irregularities, not shown, on the outer surface of the shank 24 are flattened to provide good surface roughness.

When the outer circumferential surfaces of the burnishing rollers 116 a, 116 b contact the shank 24, they should be brought into oblique contact with the surface to be burnished of the shank 24 (see FIG. 17), rather than being pressed against the shank 24 in a direction perpendicular to the axis of the shank 24 (see FIG. 16). The angle (θ2) at which the burnishing rollers 16 a, 16 b are brought into oblique contact with the shank 24 may range from about 20 degrees to 45 degrees to avoid a dent 46 (see FIG. 16) which would otherwise be formed in a plastic surface if the burnishing rollers 116 a, 116 b are pressed perpendicularly against the shank 24.

After the burnishing rollers 16 a, 16 b are linearly positioned across the shank 24 therebetween and start rollingly pressing the shank 24, one of the burnishing rollers 116 a (116 b) and the other burnishing roller 116 b (116 a) are caused to slide in respective opposite directions (directions away from each other parallel to the axis of the crankshaft 18) to rollingly press the region to be burnished of the shank 24.

As described above, after the central area of the region to be burnished is gripped by the burnishing rollers 116 a, 116 b, the burnishing rollers 116 a, 116 b are displaced in respective directions away from each other to burnish the region to be burnished. Consequently, the burnishing time is shortened for higher efficiency.

Since the burnishing rollers 116 a, 116 b are caused to slide in respective opposite directions, the round corners 48 on the both axial ends of the shank 24 as the region to be burnished can also appropriately be rollingly pressed by the round corners 28 of the first ridges 120 a (or the fourth ridges 120 d) of the burnishing rollers 116 a, 116 b (see FIGS. 13 and 14).

As shown in FIG. 14, the burnishing rollers 116 a, 116 b may not be disposed in mutually opposite relation to each other linearly perpendicular to the axis of the shank 24, but may be displaced toward the shank 24 of the crankshaft 18 from respective positions that are slightly offset horizontally to grip the shank 24. Thereafter, the burnishing rollers 116 a, 116 b may be caused to slide in respective directions away from each other to burnish the region to be burnished.

According to the present embodiment, the burnishing rollers 116 a, 116 b are disposed in mutually opposite relation to each other across the shank 24 of the crankshaft 18, and while the crankshaft 18 is rotating, the shank 24 of the crankshaft 18 is burnished by being gripped and rollingly pressed by the burnishing rollers 116 a, 116 b. Consequently, the surface properties of the shank 24 of the crankshaft 18 which is free of fillets are improved, and the crankshaft machining apparatus is simplified in structure and less costly to manufacture.

According to the present embodiment, furthermore, the first through fourth ridges 120 a through 120 d having the respective crests spaced at equal intervals along the axis of the burnishing roller 116 a are disposed on the outer circumferential surface of the burnishing roller 116 a (116 b), and the distance S by which the crests of adjacent two of the first through fourth ridges 120 a through 120 d are spaced is greater than the inside diameter T of the oil hole 126. Therefore, even if the oil hole 126 is present in the region to be burnished, the first through fourth ridges 120 a through 120 d do not fall into the oil hole 126, and the surrounding edge of the oil hole 126 is not damaged.

According to the present embodiment, consequently, even if the oil hole 126 is present in the region to be burnished, the region to be burnished can appropriately be burnished to provide good surface roughness. 

1. A method of machining a crankshaft, comprising the steps of placing a pair of burnishing rollers in opposite relation to each other across a shank of a crankshaft therebetween, gripping the shank of the crankshaft with outer circumferential surfaces of said burnishing rollers, and rotating said crankshaft and said burnishing rollers to rollingly press said shank.
 2. A method according to claim 1, wherein after said burnishing rollers are displaced toward the shank of the crankshaft and grip a substantially central area of said shank, said burnishing rollers are caused to slid in respective directions away from each other along an axial direction of said crankshaft to rollingly press said shank.
 3. A method according to claim 1, wherein said shank is burnished by being rollingly pressed prior to a heat treatment of said crankshaft including at least a high-frequency heat-treatment.
 4. An apparatus for machining a crankshaft, comprising: holding mechanisms for holding both axial ends of a crankshaft so that the crankshaft is rotatable; and a pair of burnishing rollers disposed in opposite relation to each other across a shank of the crankshaft therebetween; wherein the shank of the crankshaft is gripped by outer circumferential surfaces of said burnishing rollers, and said crankshaft and said burnishing rollers are rotated to rollingly press said shank.
 5. An apparatus according to claim 4, wherein each of said burnishing rollers is of a substantially disk-like identical shape, and the outer circumferential surface for rollingly pressing said shank comprises a single flat surface.
 6. An apparatus according to claim 4, wherein said burnishing rollers are disposed in mutually opposite relation to each other linearly perpendicular to the axis of the crankshaft.
 7. An apparatus according to claim 4, wherein said burnishing rollers are disposed in opposite relation to each other at respective positions that are offset in opposite directions by equal distances parallel to the axis of said crankshaft from a linear position perpendicular to the axis of said crankshaft.
 8. An apparatus according to claim 4, wherein each of said burnishing rollers is cantilevered on one side thereof by a support member or supported on both sides thereof by support members.
 9. An apparatus according to claim 5, wherein the outer surface of each of said burnishing rollers for contacting the shank of said crankshaft has a round corner having a radius (r2) of curvature smaller than a radius (r1) of curvature of round corners on both axial ends of said shank.
 10. A crankshaft burnishing roller for burnishing a shank of a crankshaft by rollingly pressing the shank, comprising: a pair of burnishing rollers disposed in opposite relation to each other across the shank of the crankshaft therebetween; each of said burnishing rollers being of a substantially disk-like identical shape and having an outer circumferential surface for rollingly pressing said shank, said outer circumferential surface having a plurality of ridges having respective crests axially spaced at equal intervals; said crests of adjacent two of said ridges being spaced from each other by a distance S which is greater than the inside diameter T of an oil hole defined in said shank.
 11. A crankshaft burnishing roller according to claim 10, wherein said ridges comprise at least three ridges.
 12. A crankshaft burnishing roller according to claim 10, wherein the outer surface of the ridges for contacting the shank of said crankshaft has a round corner having a radius (r2) of curvature smaller than a radius (r1) of curvature of round corners on both axial ends of said shank. 